This invention relates generally to dynamoelectric machines and more particularly to permanent split capacitor single phase electric motor systems which are capable of balanced operation at more than one load point.
Permanent split capacitor single phase motors are known to be useful in fan drive systems. Recent designs of high performance heat pumps have evolved toward heat pumps that have a dual capacity compressor system. Such a system would typically have a 2:1 capacity ratio. With this ratio, comfort conditions would require that the low capacity indoor air flow be approximately one-half to two-thirds of the high capacity air flow. Current high efficiency single phase permanent split capacitor motors when designed to operate with a deep reduction in speed using a booster winding in series with the main winding, suffer from very serious loss in motor efficiency at low speed. This is especially significant since in a heat pump application, the blower motor may operate as many as 6,500 hours per year with perhaps 4,000 of those hours in the low speed mode. Although some relief in lifetime inefficiency can be obtained by using a somewhat higher rotor resistance than would be used with a normal high efficiency motor, low speed efficiency can be further improved by operating the motor as a balanced motor at the low speed design point without sacrificing the ability to operate it as a balanced motor at the high speed operating point.
It is understood in the art that permanent split capacitor single phase motors operate very efficiently in the balanced operation mode. For example, P. H. Trickey, in "Design of Capacitor Motors for Balanced Operation", Transactions AIEE, 1932, pages 780 to 785, described a theoretical method of obtaining balanced operating conditions for single phase powered capacitor motors. The design of a permanent split capacitor single phase motor is discussed in detail in U.S. Pat. No. 4,100,444, issued July 11, 1978 to Boyd.
Permanent split capacitor motors are generally made to operate at reduced speed by adding in series with the main winding a so-called booster winding which, in conjunction with the main winding, reduces the air gap flux significantly. Current design methodology permits the design of a motor at one operating point for balanced operation. Under balanced conditions, the magnetomotive forces of the main and auxiliary windings are equal in magnitude and displaced in time by 90.degree.. This condition leads to only a forward rotating field in the air gap of the machine. The absence of a backward rotating field greatly improves the motor efficiency at this operating point. If a two-speed motor is required to perform a deep speed reduction such as required in a heat pump application, after having been designed for balanced operation at the high speed, its slow speed operation would be very badly unbalanced if conventional booster winding technology is used. Improvement in low speed efficiency can be effected by operating the main winding and auxiliary circuit at different voltages which are selected to result in balanced operation at the low speed.